According to the analysis of power fire accidents, there are 2 types and 4 points. Type One: it is the flame spread from the external firer to the cables. According to the statistics, this kind of fire only accounts 5%. The external flame lights up the cable coating and produces large amount of heat and smoke. The insulating materials, fillers and coating of the regular cables are the major combustible materials, such as PVC jackets, rubber and insulating oil. Usually the PVC jacket releases a certain amount of inflammable gas when it reaches 150 degrees Celsius, without any smoke; when it reaches 270 degrees Celsius, a large amount of inflammable gas and smoke as well as poisonous gas are released. When the temperature is over 270, it is extremely unstable and flammable. The spontaneous ignition happens when it reaches 390. But here the external fire is the cause and with a large amount of combustible gas, it will burn. For fire-retardant cables, it is still possible to have the flame spread. Different from the regular cables, the combustible temperature is up to 480 degrees Celsius. When a certain amount of gas is produced, the temperature goes up to 190, without smoke. But when there is a large amount of combustible gas, the temperature is still less than 270 degrees Celsius.

Type Two: fire accident is caused by the failure of cable. This accounts more than 95% of the fire accidents. The basic reasons are as following:

1)   The cable joint is roughly made, with poor quality, loose joint and high contact resistance. When it is exposed in the air for too long time, the insulator is getting humid and the temperature of the joint is getting higher due to long-term operation. At last the cable joint explodes and fire accident happens when the insulator is overheated.

2)   The internal flame is mainly due to the overloaded current transmission, large impedance of the joint, aging insulation and partial discharge, which leads to the temperature rise on the surface of the cable, smoldering of cable insulator and protector as well as a large amount of heat and combustible gas. As the temperature rises further, smoke is formed and then it causes serious cable fire accident.

3)   The insulation of the cable is lower and the materials are aging, which leads to the short-circuit fire accident. Especially water accumulates in the tunnel all the time; if not handled properly, a lot of cables are soaked in the water for a long time. As a result, the coating is damaged or rotten and it is affected with damp. The breakdown causes the fire accident.

4)   The heat of the cable causes the fire accident. When it is overloaded in a long run and the cooling condition of the environment is poor, it may lead to the explosion of cable joints and causes the cable fire. Whatever is the reason of the cable fire, there is a process until it happens. The breakdown point is the temperature that goes from low to high, rather than the sudden change of the current. Moreover, the power supply system in China is neutral-point unearthed. So when single-phase grounded failure happens, the cables are still able to working for several hours. The on-duty keeper couldn't find it timely and cut off the power. The flame spreads and causes the fire in a large area.